1. Field of the Invention
The present invention relates to a spark plug for use in an internal combustion engine such as an automobile engine.
2. Description of the Related Art
A spark plug for the purpose of igniting, for example, a gasoline engine for automobile, is attached on a cylinder head of the engine by means of a screw portion formed in a metal shell. A spark discharge gap formed by a ground electrode and a center electrode is located within a combustion chamber in this attached state to ignite a fuel-air mixture. Here, an electrode portion for forming the spark discharge gap is subjected to a combustion mixture gas during engine operation, and heated to considerably high temperature. Recently, a suction valve and an exhaust valve in the combustion chamber occupy larger areas along with the increasingly higher output of the internal combustion engine for use with the automobile. Therefore, it is required to reduce the size of the spark plug for use to ignite the mixture gas, and the temperature inside the combustion chamber tends to rise more highly owing to the operation of a supercharger such as a turbo charger.
In order to maintain a full life of the spark plug under the severe service conditions, it is required that the heat radiation (heat release) of the electrode portion is effected to sufficient extent. The heat of the spark plug is radiated via various passages, but in particular in a passage leading from an insulator through the screw portion of the metal shell to a cylinder head, a large quantity of heat flow will escape, and this passage plays an important role for effecting the heat radiation. In a commonly used spark plug, the length (reach) of this screw portion has a maximum value of at most about 19 to 20 mm, but lately an attempt of improving the heat radiation performance of the spark plug has been made by further lengthening this thread reach.
By the way, as the screw portion is made a long reach, the insulator made of ceramic such as alumina is obliged to be longer. In this case, there is the problem that if any impact or excessive torque is exerted in attaching the spark plug, the insulator is likely to fracture or crack. For example, in a case of a spark plug having a resistor incorporated into the insulator, the resistor is disposed through a through hole of the insulator between the terminal and the center electrode, but when a bending force is applied to the insulator, the top end edge of the terminal located within the through hole is liable to act as a support against fracture, resulting in the problem that the insulator is more likely to break.
It is an object of the present invention to provide a spark plug which is able to maintain the breaking strength of the insulator even if the screw portion is lengthened, and is structured less likely to cause inconvenience such as breakage of the insulator, when the spark plug is attached.
In order to solve the above-described problems, a spark plug according to the present invention comprises an axial center electrode, an axial insulator covering the outside of the center electrode, a metal shell shaped like a barrel that is open at both ends and disposed outside the center electrode, and a ground electrode for forming a spark discharge gap with respect to the center electrode, the ground electrode connected to the metal shell, characterized in that the forward side of the insulator is defined as the side where the spark discharge gap is located in an axial direction of the insulator, the backward side being defined as the opposite side, a screw portion portion has a thread reach of 25 mm or greater on an outside circumferential surface located at the forward end portion of the metal shell, and a portion of the insulator located within the metal shell half way in the axial direction has a peripheral flange portion protruding outward and a middle trunk portion adjacent the forward side of the flange portion, wherein a through hole is formed in the axial direction of the insulator, a terminal is secured at the rear end side of the insulator, the center electrode is secured at the front end side of the insulator, and an electrically conductive binder layer is disposed between the terminal and the center electrode within the through hole, the terminal having the top end edge located with getting into the middle trunk portion of the insulator, and wherein the wall thickness of the middle trunk portion is determined to satisfy the relation such as,
0.42xe2x89xa6(Dxe2x88x92d)/Dxe2x89xa60.79 
where the outer diameter of the middle trunk portion at a position corresponding to the top end edge of the terminal is D, and the inner diameter of the through hole in the middle trunk portion is d.
In a typical spark plug, an insulator 201 has a flange-like large flange portion (referred to as a flange portion) 201a formed to be caulked to a metal shell 200, and a middle trunk portion 201b closer to the top end of the insulator 201, as shown in FIG. 4A. In the spark plug having a thread reach of 20 mm or less, a terminal 202 is adjusted in length to have its top end edge within the flange portion 201b. On the other hand, with a larger thread reach, there is the need of lengthening the middle trunk portion 201b of the insulator 201. However, since the length of a resistor or an electrically conductive binder layer 203 such an electrically conductive glass seal layer can not be extended freely owing to the restrictions from its electrical characteristics or production conditions, a way of extending the top end portion of the terminal 202b is employed.
As a result, in the case where it is necessary to have a structure in which the top end portion of the terminal 202b extends into the middle trunk portion 201b, the top end edge of the terminal 202, which is located within the middle trunk portion 201b that is thinner than the flange portion 201a, acts as a support against fracture, when a bending force is exerted externally, and there is more likelihood of causing the crack C, as shown in FIG. 4B. In particular, in a spark plug of long reach type having a thread reach of 25 mm or greater according to the present invention, the length of the middle trunk portion 201b is necessarily longer, so that a larger bending moment is caused by application of an outside force and exerted on a support against fracture, resulting in the severe problem such a breakage. Thus, in the present invention, the wall thickness of the middle trunk portion at a position corresponding to the top end edge of the terminal is sufficiently set to be 0.42 or larger, based on the previous value of (Dxe2x88x92d)/D, whereby the endurance strength against the bending of insulator or impact thereon is remarkably improved, and further, it is possible to prevent the inconvenience such as fracture of the insulator when the spark plug is attached. However, if the value of (Dxe2x88x92d)/D is beyond 0.79, the inner diameter d of the through hole is too small to secure the thickness of center electrode fully, leading to malfunction of the spark plug to cause degraded heat releasing characteristic. Note that the value of (Dxe2x88x92d)/D is preferably set in the range from 0.43 to 0.60.
FIGS. 4A and 4B is a typical view illustrating a common structure of the spark plug, but not representing the public nature of the constitutional elements of the present invention.